trace: Allow specifying a new span's parent (#923)

This branch allows users of `tokio-trace` to explicitly set a span's
parent, or indicate that a span should be a new root of its own trace
tree. A `parent: ` key has been added to the `span!` macros. When a span
is provided, that span will be set as the parent, while `parent: None`
will result in a new root span. No `parent:` key results in the current
behaviour.

A new type, `span::Attributes`, was added to `tokio-trace-core` to act
as an arguments struct for the `Subscriber::new_span` method. This will
allow future fields to be added without causing breaking API changes.
The `Attributes` struct currently contains the new span's metadata,
`ValueSet`, and parent.

Finally, the `span::Span` type in `-core` was renamed to `span::Id`, for
consistency with `tokio-trace` and to differentiate it from
`span::Attributes`. This name was chosen primarily due to precedent in
other tracing systems.

Closes #920 

Signed-off-by: Eliza Weisman <[email protected]>

tokio-trace/examples/counters.rs

@@ -77,8 +77,8 @@ impl Subscriber for CounterSubscriber {
         interest
     }
 
-    fn new_span(&self, _new_span: &Metadata, values: &field::ValueSet) -> Id {
-        values.record(&mut self.recorder());
+    fn new_span(&self, new_span: &span::Attributes) -> Id {
+        new_span.values().record(&mut self.recorder());
         let id = self.ids.fetch_add(1, Ordering::SeqCst);
         Id::from_u64(id as u64)
     }